Periodic table with charges

What is an ion charge?

An atom is neutral because it has equal numbers of protons and electrons. When it loses electrons it becomes a positive ion (a cation), and when it gains electrons it becomes a negative ion (an anion). The charge is the number of electrons lost or gained: a sodium atom that loses one electron becomes Na⁺, with a charge of +1, and a chlorine atom that gains one becomes Cl⁻, with a charge of −1.

Atoms usually lose or gain just enough electrons to reach the same electron arrangement as the nearest noble gas. That full outer shell is especially stable, which is why charges follow the group number so closely.

How to find the charge from the group

For main-group elements the group gives the charge. Group 1 metals form +1 ions, group 2 metals form +2 ions and aluminum in group 13 forms +3. On the right side of the table the pattern reverses for the nonmetals and metalloids: those in group 15 take −3, those in group 16 take −2 and the halogens in group 17 take −1, because gaining three, two or one electron completes their outer shell. Metals lower in these groups, such as bismuth (+3, +5), form positive ions instead. Group 18 is shown as 0, since the noble gases already have a full shell.

Carbon, silicon and germanium in group 14 are shown as ±4, and boron as +3, the charges that follow from their group. In practice these elements, like many nonmetals, usually share electrons in covalent bonds instead of forming simple ions. Hydrogen is shown with two charges: +1, as in acids, and −1 in metal hydrides such as sodium hydride (NaH).

Transition metals and other exceptions

Transition metals do not follow a simple group rule. They lose their outer s electrons first and can then lose some d electrons as well, so many of them form more than one ion. Iron is shown as +2, +3 and copper as +1, +2, while tin and lead, two post-transition metals, are shown as +2, +4. Zinc (+2) and silver (+1) are useful exceptions with a single common charge. When a metal has more than one charge, the charge goes in the name as a Roman numeral: iron(II) chloride is FeCl₂ and iron(III) chloride is FeCl₃.

For the superheavy elements from meitnerium (109) to oganesson (118), only a few atoms have ever been made, so their charges are unknown and appear as a dash. Charges are not the same as oxidation states, which also cover covalent compounds; the oxidation states view lists those.

Worked example: aluminum oxide

Aluminum is in group 13 and is shown as +3. Oxygen is in group 16 and is shown as −2. In an ionic compound the positive and negative charges must cancel out, and the smallest numbers of ions that do this are two aluminum ions and three oxide ions: 2 × (+3) + 3 × (−2) = 0. The formula of aluminum oxide is therefore Al₂O₃. The same method works for any pair of ions, such as magnesium (+2) and chlorine (−1), which give MgCl₂.